Cotton and other cellulose scraps produced when cutting cotton fabrics during clothing manufacture are a waste product typically buried in landfills or consumed in incinerators. Garnetting or other maceration techniques to separate and recover the cotton fibers from the scraps shortens the fiber lengths, and the resulting products have few end uses. As a consequence, over 200 million pounds of denim scrap alone is destroyed as waste each year.
U.S. Pat. Nos. 5,376,143 and 5,471,720 describe a process for recycling denim waste by separating the fibers, preparing a colored yarn of a blend of the recycled fibers and virgin fibers, and preparing denim or similarly dyed fabric from the yarn. This process has not been commercially implemented, perhaps because of costs of fiber separation and the limitations of the shortened fibers in making a strong, durable fabric.
Many applications of cotton, however, do not require long fibers. Cotton batting is a popular absorbent because of its softness and cushioning characteristics and high water absorbency. It is a preferred component for many industrial and household products, such as quilts, upholstery, sanitary napkins and diapers, and medical products such as swabs, bandages and the like. However, most of these applications require that the cotton fibers be purified, colorless, and strong, and a process for recycling cotton scrap to produce cotton fibers for these applications has not been commercially feasible because of the difficulties in processing the scrap. One principal area of difficulty is removal and/or decolorizing of the vat dyes present in many cotton scraps such as denim.
Vat dyes consist of solubilized colored compounds which are usefully precipitated as the insoluble form within cellulosic fibers. These compounds are reversibly changed to a water-soluble "leuco" state by chemically reducing them in an alkaline reduction process. This is done easily by mixing the dye into a water solution containing a water-soluble reducing agent such as alkaline sodium hydrosulfite. In a dying process, the cellulosic fiber is typically immersed in a bath containing such a reduced leuco solution, and the dye is allowed to penetrate the substrate.
After this immersion, the fiber is exposed to an oxidizing environment. Such an environment is air and, in one such process, the yarn, wetted in a leuco solution, is draped in long beams over rolls and exposed to air until the dye and accompanying reducing agents are oxidized. Dilute hydrogen peroxide or another peroxygen compound can also used for this oxidation. In each case, the oxidized medium converts the leuco dye into its original water-insoluble state. If the dye molecule is contained within the cellulose substrate, the water-insoluble dye is trapped and cannot be removed by casual exposure to water and detergents.
Fabric is often dyed with more than one leuco dye. It common practice to dye dark shades of indigo first with the leuco form of a black sulfur dye and second with the leuco form of blue indigo dye. Both dyes require subsequent oxidation to render them water-insoluble.
Since the vat dyeing process involves oxidation to the leuco state to form the insoluble oxidation product, vat dyes are chosen to be resistant to the action of bleaching oxidants. Vat-dyed cellulose textiles must be stable in the presence of hydrogen peroxide and are resistant to hydrogen peroxide oxidation to a colorless or bleached state. Bleaching oxidation of vat-dyed cellulose textiles has therefore required the application of stronger oxidants such as sodium hypochlorite, for example, which also attack and weaken the fibers under the harsh conditions required to achieve complete decolorization. The halide bleaches also create serious environmental problems in the volumes required for commercial applications.
A portion of the vat dyes can be removed and the dyes recovered by the reducing process described in U.S. Pat. No. 5,366,510, for example, the contents of which is hereby incorporated by reference. Removal and recovery of vat dyes from denim fabric scraps is advantageous in the recycling process. However, only a portion of the dye can be removed by this process, and a process is needed which can remove or eliminate the residual coloring in the product without significantly reducing the strength and other properties of the cellulose fibers.
As will be more evident from the description of the invention hereinafter, this invention is based on a discovery that catalyzed hydrogen peroxide bleaching processes which have heretofore been applied to bleaching paper pulp are sufficiently strong to overcome the hydrogen peroxide oxidation resistance of the dyes and to effectively remove and eliminate vat dye coloring from cellulosic textiles and textile scraps. But, in contrast to the strong bleaches, the process does not cause significant change to the strength or other qualities of the cellulose fibers. The process furthermore has the advantage of being environmentally safe. Obviously, other oxidation-resistant dyes from other dye classes would benefit from this process.